The present invention relates to a projection exposure apparatus and a projection exposure method. More specifically, the present invention relates to a projection exposure apparatus and a projection exposure method both useful for projection exposure of a pattern defined on a mask onto a substrate in the manufacture of a semiconductor device, a liquid crystal display device or the like by a photolithography process.
In a projection exposure apparatus used in a photolithography process for the manufacturing a semiconductor device, a liquid crystal display device, a thin-film magnetic head or the like, a circuit pattern with fine and narrow line width formed on a mask (e.g., a reticle) is projectionally exposed onto a photosensitive substrate (e.g., a wafer; hereinafter, a xe2x80x9cwaferxe2x80x9d is used an exemplary substrate for the description below) with a resist layer applied thereon, via a projection optical system with a high resolution. A wafer stage is moved vertically (i.e., upward and downward) along the optical axis of the projection optical system to align the wafer surface on the wafer stage to the focal position of the projection optical system. The wafer stage is then moved in specific directions two-dimensionally on a plane orthogonal to the optical axis of the projection optical system. For example, in a step-and-repeat type of projection exposure apparatus, the wafer stage is stopped at such a position that the center of the exposure field (i.e., the optical axis) of the projection optical system coincides with the center of each shot region on the wafer, where the exposure operation is performed. In such a projection exposure process, a photosensitive resin (e.g., a novolak resin) is usually applied on the wafer as the resist layer.
Recently, in such a type of apparatus, for addressing the demand for a more miniscule circuit patterns, the resolution of the apparatus has been improved. For this purpose, a light with a short wavelength in the UV zone has been generally used as the exposure illumination light. It has also been demanded to improve the workability of the projection exposure apparatus, which depends on the exposure time period. For this purpose, an exposure illumination light with a high illuminance has been used to shorten the exposure time period. Therefore, for pattern transfer, the surface of the substrate (e.g., a wafer, a glass plate) applied with a photosensitive agent (e.g., a photoresist) is irradiated with a light with a high energy density.
When a light with a high energy density is irradiated onto the surface of the substrate applied with a photosensitive agent, so-called xe2x80x9cablationxe2x80x9d may occur in which the photosensitive agent or a reduction product thereof (hereinafter, referred to as a xe2x80x9cphotosensitive agent or the likexe2x80x9d) is scattered or vaporized and then evaporated. During the projection exposure process, the photosensitive agent released from the substrate partly reaches onto the optical member disposed in the vicinity of the substrate, and deposited on the surface opposed to the substrate (hereinafter, simply referred to as the xe2x80x9csubstrate-opposed surface) of the optical member, sometimes resulting in contamination of the surface of the optical member.
The deposits (e.g., a photosensitive agent or the like) on an optical member exhibit the behavior of optical contamination on the optical member. For example, in the case of an optical member as a component of a projection optical system, at the time of pattern transfer, the illuminance on an area of the substrate surface (applied with a photosensitive agent) corresponding to the contaminated area of the optical member may be decreased compared to that corresponding to the non-contaminated area of the optical member. That is, regional unevenness in amount of exposure light may occur in the pattern transferred onto the substrate surface applied with a photosensitive agent. As a result, for example, the line of a pattern designed to have an even line width may be transferred unevenly. For the manufacture of an integrated circuit pattern, such uneven pattern line transfer may ultimately cause functional failure of the finished product.
In addition, when the deposits on the optical member can absorb the illumination light, the temperature of the optical member is increased as the deposits are heated by the absorption of the illumination light during pattern transfer. Consequently, the optical characteristics of the optical member are altered, ultimately resulting in the alternation of the total imaging characteristics of the entire projection optical system.
In order to overcome this defect, in a projection exposure apparatus, it has been conventionally required to wipe the surface of an optical member of a projection optical system by an operator manually (cleaning process). It has also be proposed to provide a replaceable cover glass or polymer film between an optical member and a wafer to thereby prevent the approach of contaminants to the optical member (Japanese Patent Application Laid-open No. 6-140304).
It is the recent trend to increase the numerical aperture (NA) of a projection optical system to the maximum for the purpose of increasing the resolution of the system to the maximum. In this case, the maximum of the incident angle of the exposure illumination light upon the wafer is more increased. Then, it has been proposed to bring the projection optical system close to the wafer at the time of pattern transfer, thereby decreasing the diameter of the optical system of the exposure apparatus. This technique is also beneficial for the reduction in aberration for optical design of the exposure apparatus.
However, when the projection optical system is brought closer to the wafer at the time of pattern transfer, the deposition of contaminants (e.g., a photosensitive agent and the like) are released from the wafer onto the optical member of the projection optical member. Thus, the contamination of the optical member in the projection optical system is a serous problem. Moreover, the use of an exposure illumination light with a shorter wavelength and a higher illuminance for the purpose of improving resolution and workability of the apparatus makes this problem more serious.
In a prior art technique for disposing a replaceable contamination protection member between a projection optical system and a wafer, the space between the projection optical system and the exposure surface of the wafer has a three-layered structure, at the time of pattern transfer, which consists of a layer of the atmosphere between the projection optical system and the contamination protection member, a layer of the contamination protection member, and a layer of the atmosphere between the contamination protection member and the exposure surface of the wafer. Therefore, there is a limitation in bringing the projection optical system close to the wafer at the time of pattern transfer.
In addition, because of the increased demand for high transfer accuracy, the installation repeatability for a replaceable contamination protection member and the permissible optical variability of the contamination protection member become very critical. Therefore, the application of a cover glass (which has an even thickness) as the replaceable contamination protection means has its limit. That is, when a contamination protection member is replaced with a new one, it is inevitable to re-adjust the total aberration balance of the projection optical system; which is not suitable for practical use.
When a polymer film is used as the replaceable contamination protection member, the re-adjustment of the aberration balance of the projection optical system is not necessary. However, among the components of the exposure illumination light, the transmittance on the film of components having a large incident angle upon a polymer film are decreased. Particularly, the transmittance of an S-polarized light (which is largely responsible for an imaging characteristic) is remarkably increased, resulting in poor imaging performance. Moreover, a polymer film is generally difficult to be applied with an anti-reflection coating; which is not also suitable for practical use.
Under these circumstances, the present invention has been accomplished. The first object of the projection exposure apparatus according to the present invention, therefore, is to provide a projection exposure apparatus with improved precision by reducing the contamination caused by the deposition of a photosensitive agent or the like on an optical member of a projection optical system or the like.
The second object of the projection exposure apparatus according to the present invention is to provide a projection exposure apparatus with improved accuracy, by cleaning an optical member of a projection optical system or the like at the time when pattern transfer is not performed.
The third object of the projection exposure apparatus according to the present invention is to provide a projection exposure apparatus with improved accuracy by inspecting an optical member of a projection optical system or the like for contamination and, based on the inspection result, cleaning or replacing the optical member at the time when pattern transfer is not performed.
The fourth object of the projection exposure apparatus according to the present invention is to provide a projection exposure apparatus with improved accuracy by protecting an optical member of a projection optical system or the like for contamination during pattern transfer is performed.
The object of the projection exposure method according to the present invention is to provide a projection exposure method for performing the pattern transfer operation satisfactorily under the condition where the contamination caused by the deposition of a photosensitive agent or the like on an optical member of a projection optical system is reduced.
The invention is directed to a projection exposure apparatus for transferring a pattern defined on a mask onto a wafer via a projection optical system, the apparatus including a cleaning device for cleaning an optical member disposed at a given position.
According to the projection exposure apparatus of the present invention, the contamination of the substrate-opposed surface of the optical member at the time of pattern transfer can be reduced and the exposure accuracy can be improved by, prior to the pattern transfer, removing the contaminants (e.g., a photosensitive agent), caused by the previous pattern transfer, on the substrate-opposed surface of the optical member arranged at the predetermined position in the vicinity of the exposure surface of the substrate.
The cleaning may be performed every time pattern transfer is performed. However, if the contamination of the optical member caused by one pattern transfer is very small, then the cleaning may be performed at any time when the contamination level falls within a predetermined permissible range, for example, every time pattern transfer is performed for a predetermined number of times, or at regular intervals.
In the projection exposure apparatus of the present invention, various configurations may be employed. For example, the cleaning device may be disposed on a stage for holding the substrate, or the cleaning device may be mounted on a movable mechanism distinct from the stage for holding the substrate.
According to the projection exposure apparatus of the present invention, the cleaning member necessary for the cleaning operation can be driven by a conventionally employed stage which is for holding the substrate and is used for moving the substrate for performing pattern transfer. Therefore, the cleaning operation can be performed in a simple manner.
In the projection exposure apparatus of the present invention, it is preferable that the cleaning device be mounted on a vertical movement drive device provided on the stage.
According to the projection exposure apparatus of the present invention, the cleaning device and the optical member can be placed in contact with each other or [separate] can be separated from each other by a drive device.
According to the projection exposure apparatus of the present invention, the cleaning device is moved to the cleaning position by a movement mechanism distinct from the stage for holding the substrate. Therefore, both the cleaning function and the pattern transfer function are performed while maintaining the accuracy and the cleanliness of the stage for holding the substrate.
According to the projection exposure apparatus of the present invention, various types of cleaning devices may be employed. For example, the cleaning device may comprise an ultrasonic cleaning device for ultrasonically cleaning a part to be cleaned of the optical member by immersing the part in a cleaning solution with ultrasonic vibration. Alternatively, the cleaning device may comprise a cleaning member which contacts the substrate-opposed surface of the optical member to the substrate-opposed surface or the cleaning device may comprise a solution injector for emitting a cleaning solution onto the substrate-opposed surface of the optical system.
In the projection exposure apparatus of the present invention, it is preferable that the cleaning device is a soft member which has a surface to be placed in contact with the substrate-opposed surface so that it causes no damage to the optical member when they are in contact with each other. The cleaning device may be further provided with a drive unit for driving the cleaning member so that the substrate-opposed surface contacts the soft member so that the substrate-opposed surface is rubbed against the soft member. As such a soft member, a porous material capable of being impregnated with a cleaning solution may be used.
In the projection exposure apparatus of the present invention, it is preferable to employ, as the solution injector, an injector capable of emitting the cleaning solution applied with ultrasonic vibration.
On the other hand, an optical cleaning device for irradiating an irradiation light having an optically cleaning effect onto the surface of the optical member can be used.
The invention is further characterized in that the optical cleaning device comprises a gas supply means for supplying an oxidation-enhancing gas in the vicinity of the surface of the optical member. Therefore, the optical cleaning effect can be improved, since an oxidation-enhancing gas is supplied in the vicinity of the surface of the optical member during the cleaning operation by the cleaning device.
The invention is further characterized in that the optical cleaning device comprises a shield means for shielding the atmosphere including the optical path of the irradiation light in the vicinity of the surface of the optical member, against the external environment. Therefore, the cleaning effect can be increased by the action of the oxidation-enhancing gas, since the atmosphere in the vicinity of the surface of the optical member is shielded from the external environment by a shield means in such a way that includes the optical path of the irradiation light.
The invention is further characterized in that the optical cleaning device is one capable of irradiating an irradiation light onto the optical member through a replaceable window. Therefore, the optical cleaning device can be protected by the window, and the optical cleaning effect can be satisfactorily maintained by replacing the window with a new one if the window is contaminated.
The invention is further characterized in that the apparatus includes a contamination inspection device for inspecting the optical member for contamination. The invention is further characterized in that the cleaning device comprises a judging unit for determining whether or not cleaning of the optical member is necessary based on the inspection result obtained in the contamination inspection device.
The present invention is further directed to a projection exposure apparatus for transferring a pattern defined on a mask onto a substrate via a projection optical system, wherein there is provided a contamination inspection device for inspecting the optical member disposed in a given position for contamination. The contamination inspection device may be provided singly or in combination with the cleaning device.
When the contamination inspection device is provided in combination with the cleaning device, prior to the pattern transfer operation, the optical member may be inspected for contamination to determine whether or not the cleaning of the optical member is necessary. Based on the inspection result obtained in the contamination inspection device, it can be ensured that the cleaning operation for the optical member is performed at any time when it is required, and the cleaning effect can be confirm. Thus, the exposure accuracy can be improved.
When the contamination inspection device is provided singly, prior to the pattern transfer operation, the optical member may be inspected for contamination to determine whether or not the replacement of the optical member is necessary. Based on the inspection result obtained in the contamination inspection device, it can be ensured that the optical member is replaced with a new one at any time when it is required. Therefore, the exposure accuracy can be improved.
For the contamination inspection device, various types of constructions may be possible. For example, the contamination inspection device may comprise: an irradiation optical system for irradiating light to the optical member; a photodetector for detecting the light from the optical member; and a contamination inspection processing unit for inspecting the optical member for contamination based on the detection result from the photodetector.
As the inspection light for the inspection, an exposure illumination light may be available. In this case, it is required to chose an optical member capable of transmitting an UV light as the optical member of the inspection optical system. Such a requirement limits the range of choices for the optical member. Therefore, it is preferable that the contamination inspection device be adapted to irradiate the inspection light onto the surface facing the substrate of the optical member, and that the inspection light detect a spectrum of a reflection component reflected on the optical member.
The invention is further characterized in that an inspection means for determining the contamination level of the surface of the optical member disposed in the vicinity of the projection optical system of the projection exposure apparatus. Therefore, the contamination level of the surface of the optical member at the vicinity of the substrate in the projection optical system is determined by the inspection means. Based on the determined result, before the cleaning operation, the necessity of the cleaning operation can be determined, while after the cleaning operation, the contamination removal level by the cleaning operation can be known.
The invention is further characterized in that the inspection device measures a reflectance of a reflection light reflected on the surface of the optical member or a transmittance of a transmission light passed through the surface of the optical member, and then determines the contamination level of the surface of the optical member based on the measurement result. Therefore, the inspection for contamination can be performed by determining the reflectance or transmittance on the surface of the optical member to be inspected.
The invention is further characterized in that the inspection device determines the contamination level of the surface of the optical member based on the comparison result between a preset given reflectance of transmittance and an actually measured real reflectance or transmittance, respectively. The predetermined reflectance or transmittance may be defined as a reflectance or transmittance on the surface of the optical member which is determined when the contamination level of the surface of the optical member falls within a predetermined permissible range. Therefore, the contamination level can be determined in such a manner that a reflectance or transmittance on the surface of the optical member to be inspected is previously determined (which is defined as the predetermined reflectance R0 or transmittance), and the contamination level is then determined based on the difference between an actually determined real reflectance Rr or real transmittance and the predetermined reflectance R0 or transmittance, respectively.
The invention is further characterized in that the inspection device measures a reflectance of a reflection light reflected on the surface of the optical member or a transmittance of a transmitted light passed through the surface of the optical member, and then determines a real reflectance Rr or transmittance from the comparison result between a photoelectric signal for the irradiation light received after reflected on the surface of the optical member or after passed through the surface of the optical member and a photoelectric signal for the irradiation light received without the presence of the optical member. Therefore, for example, when the irradiation light is irradiated for several times at intervals during the cleaning operation, the variation with every cleaning operation in contamination removal level can be reflected as the real reflectance Rr or transmittance.
The invention is further characterized in that both the above-mentioned contamination inspection device and the above-mentioned cleaning device for the optical member are provided. The invention is further characterized in that the cleaning device comprises an optical cleaning device for irradiating an irradiation light having an optical cleaning effect onto the surface of the optical member. Therefore, when it is determined that xe2x80x9cthe cleaning operation is necessaryxe2x80x9d based on the result obtained in the projection optical system inspection device, a given irradiation light is emitted from the optical cleaning device to remove the contamination on the surface of the optical member.
The present invention is also directed to a projection exposure apparatus for transferring a pattern defined on a mask onto a substrate via a projection optical system, the apparatus including a contamination protection device for making a gas flow between the optical member disposed in the vicinity of the exposure surface of the substrate and the substrate to prevent the approach of contaminants generated from the substrate toward the optical member.
According to the projection exposure apparatus of the present invention, the contamination protection device makes gas flow between the substrate and the optical member to prevent the approach of the contaminants released from the substrate toward the optical member. Therefore, the presence of any intermediate other than the gas is not needed between the optical member and the substrate on the traveling path of the exposure illumination light. Therefore, the contamination of the substrate-opposed surface of the optical member can be reduced and the exposure accuracy can be improved.
For the contamination inspection device, various constructions may be employed. For example, the contamination protection device may comprise: a flow-path setting member for setting the flow path of the gas in the vicinity of the substrate-opposed surface of the optical member; and a gas supply unit for supplying the gas to the flow path set by the flow-path setting member.
The above-described variations of the present invention may be applicable for various types of optical members. For example, the optical member may be one disposed on the optical path of the exposure illumination light in the vicinity of the substrate, or it may be one disposed at the end part of an optical system for alignment of the substrate.
As the optical system for alignment of the substrate, an alignment optical system for alignment of the substrate in the direction vertical to the optical axis of the projection optical system or a focus detection optical system for alignment of the substrate in the direction parallel to the optical axis of the projection optical system may be employed, for example.
According to the projection exposure apparatuses of the present invention, there is no need of apprehension of the contamination of the optical member which is arranged on the optical path of the exposure illumination light and in the vicinity of the substrate caused by the attachment of contaminants during pattern transfer. In addition, it becomes possible to design a projection exposure apparatus with a projection optical system disposed close to a substrate. Therefore, a projection exposure optical system with higher numerical aperture can be readily manufactured.
According to the projection exposure apparatuses of the present invention, the optical member arranged at the end part of an optical system for alignment of the substrate, which is likely to become adhered with a photosensitive agent or the like, is the target of the cleaning, the contamination inspection or the contamination protection. Therefore, the contamination of the optical member can be reduced and the exposure accuracy can be improved.
The present invention is further directed to a projection exposure method for transferring a pattern defined on a mask onto a substrate via a projection optical system, the method comprising: a cleaning step for removing the contamination on an optical member disposed at a given position; and a pattern transfer step for transferring a pattern defined on the mask onto the substrate via the projection optical system.
The present invention may be applicable for various types of optical members. For example, the optical member may be one disposed on the optical path of the exposure illumination light and in the vicinity of the substrate.
According to the projection exposure method noted above, prior to the pattern transfer step, the cleaning step is first performed to remove the contaminants (e.g., a photosensitive agent), as a result of the previous pattern transfers, attached on the optical member disposed near the exposure surface of the substrate, thereby making the optical member clean. Therefore, the pattern transfer can be satisfactorily performed under the condition where the contamination of the optical member is reduced and any intermediate other than the atmosphere is not present between the projection optical system and the substrate.
The cleaning operation may be performed every time the pattern transfer operation is performed. However, if the contamination of the optical member caused by one pattern transfer is small, it may also be possible to perform, prior to the cleaning step, a contamination evaluation step for determining the relation between the contamination level of the optical member and the number of pattern transfer operations and then determining the limit in number of pattern transfer operations, which is the number of exposure operations during which the clean substrate-opposed surface of the optical member is contaminated to a predetermined impermissible level; and the cleaning step is performed before the number of pattern transfer operations performed after the previous cleaning step exceeds the limit.
According to the projection exposure method of the present invention, the cleaning step can be performed at any time when the contamination level falls within a predetermined permissible range, for example, every time pattern transfer is performed for a predetermined number of times or at regular intervals. Therefore, the workability of the apparatus can be improved.
The invention is further directed to a projection exposure method for transferring a pattern defined on a mask onto a substrate via a projection optical system, the method comprising: a contamination inspection step for inspecting an optical member disposed at a predetermined position for contamination; and a judging step for determining whether or not pattern transfer will be performed based on the inspection result obtained in the inspection step.
According to the just described projection exposure method, prior to pattern transfer, the contamination inspection step is performed to inspect the optical member for contamination. In the judging step, based on the result obtained in the contamination inspection step, it is determined whether or not the contamination level of the optical member caused by the pattern transfer operations exceeds a predetermined permissible limit. When it is estimated that the contamination level of the optical member exceeds the predetermined permissible limit, the cleaning or replacement of the optical member is performed. When it is estimated that the contamination level of the optical member does not exceed the predetermined permissible limit, the pattern transfer is performed. In this manner, based on the result obtained in the contamination inspection step, it can be ensured that the optical member is cleaned or replaced with a new one at any time when it is required. Therefore, the exposure accuracy can be improved and the workability of the apparatus can also be improved.
The invention is further characterized in that, in the above-noted cleaning step, an irradiation light having an optical cleaning effect is irradiated onto the contamination on the optical member. The invention is further characterized in that a cleaning step for removing the contamination on the optical member based on the inspection result obtained in the inspection step is further included.
The invention is further directed to a method for assembling a projection exposure apparatus for transferring a pattern defined on a mask onto a substrate via a projection optical system, the method comprising: providing a cleaning device for cleaning an optical member disposed at a given position relative to the projection exposure apparatus. The invention is further characterized in that the cleaning device is an optical cleaning device for irradiating an irradiation light having an optical cleaning effect onto the contamination on the optical member. The invention is further characterized in that the projection exposure apparatus is further provided with a contamination inspection device for inspecting the optical member for contamination.